The invention is applicable especially to the supply of machining machines, for example portable drilling machines having pneumatic motors.
Such a machine is connected to a source of compressed air, for example via a tube, and generally comprises a rotary spindle carrying a drilling tool.
If there is a drop in pressure upstream of the machine when it is in operation, for example as a result of the formation of a kink in the tube, the pneumatic motor no longer drives the drilling tool.
When there is a later rise in pressure, for example when the kink is removed from the tube, the tool may in some machines be driven again inadvertently by the motor.
It will be appreciated that this involves risks of injury to an operator using the drilling machine.
Safety electrovalves which are used to equip circuits for the supply of compressed air are also known. Such an electrovalve is controlled as a function of signals supplied by a pressure sensor outside the valve in order to trigger its closure when the pressure falls below a predetermined threshold value.
Such electrovalves lead to relatively complex and expensive structures which are poorly suited to portable machines.
An object of the invention is to solve those problems by providing a valve of the above-mentioned type which enables the risks of injury resulting from a momentary drop in the pressure of the fluid to be limited, and which has a simple structure and a reduced cost.